Transponder system with variable frequency transmission

ABSTRACT

An electronic identification system comprising a transmitter for generating an electromagnetic excitation signal, one or more portable transponders for storing variable identification data, and for transmitting an information signal containing the identification data upon entering the field. Transmission of the information signal is independent of the excitation signal in both time and frequency. A radio frequency receiver is provided for receiving the information signal and in response generating output signal representing the variable identification data contained in the information signal. The system overcomes two principle disadvantages associated with existing systems: and excitation frequency can be selected for optimum performance given the circumstances dictated by each application; an independent response channel can be selected according to the requirements of each installation or application, thereby allowing response signals to be transmitted by transponders at a frequency which differs form the frequency of an interfering signal.

The present invention relates in general to communication systems, andmore particularly to an electronic identification system comprising oneor more portable transponders for transmitting an information signal inresponse to entering an electromagnetic excitation field.

Electronic identification systems are well known in the art foreffecting automatic identification of objects, animals and people, andare used in situations where elementary line of sight systems such asbar codes cannot be used. Such prior art identification systemstypically comprise a plurality of electronic transponders (commonlyreferred to as electronic tags or simply tags) which are attached to theparticular objects to be identified, and at least one interrogator (thatis a controlled transmitter and receiver, sometimes referred to as areader) for exciting the transponders into transmitting response signalswhich are then detected by the interrogator, decoded and converted intoinformation for display to a human operator, or transmitted as data to acomputer.

One such identification system is described in United Kingdom patentapplication GB 2,112,607 (Senelco Limited) which discloses a systemcomprising a transponder having a receiver for receiving a transmittedsignal S1, a generator for generating a further carrier signal S2, logicmeans for modulating the signal S2 according to a coded mathematicalrelationship with the received signal S1, and a transmitter fortransmitting the signal S2. Specifically, the carrier frequency of thesignal S2 is disclosed as being a multiple or a fraction of thefrequency S1.

United Kingdom patent application GB 2,157,132 (Senelco Limited) alsodiscloses an identification system of the form discussed above inconnection with the '607 specification, but in addition incorporatescircuitry for receiving a reply signal (e.g. information signal) fromthe transponder, checking the reply signal for possible contentionbetween reply signals from different transponders which arrive at thesame time, and initiating retransmission of the reply signals in theevent such a contention situation is detected. Specifically, the tagreceives a signal from the interrogator which it compares with storedinformation in order to determine whether the message transmitted by thetag has been received by the interrogator for confirming that the taghas been identified by the interrogator and can therefore fall silent.Contention in this system is resolved by sophisticated duplexcommunications between reader and tag in order to achieve simultaneousidentification of numerous tags.

As with the '607 specification, the circuitry in the '132 disclosureincorporates means for deriving a received carrier signal at thetransponder, and using the derived carrier signal to provide atransmission signal which is related to and therefore dependent on thesustained presence of the received signal from the interrogator.

Additional prior art systems are well known for teaching transmission ofthe transponder information signals which are dependent on the receivedinterrogation signal, as follows:

European patent application 0,310,127 (Texas Instruments) teaches atransponder arrangement which transmits an information signal at apredetermined carrier frequency related to the frequency of a receivedRF interrogation pulse.

United Kingdom patent application GB 2,202,981 (Satellite Video SystemsLtd.) discloses a system incorporating a transponder designed to modifya received interrogation signal in a variable manner and then retransmitthe modified signal for reception by the interrogation unit.

United Kingdom patent application GB 2,163,324 (Electromatic) disclosesa system in which energy is extracted from a detected interrogationsignal, compared to a reference level and then supplied to the remainderof the transponder circuit for enabling generation and transmission ofthe information signal.

European patent application 0,253,368 (Amtech Corporation) discloses anidentification system comprising a reader and a transponder, thetransponder being adapted to receive an interrogation signal and inresponse generate a modulated response.

A fundamental disadvantage of such prior art systems in which theinformation signal transmitted by the transponder is related to (i.e.dependent on) the received interrogation signal, is that in situationsin which there is fast relative movement between the transponder and theinterrogator there may be insufficient time for the transponder toextract the carrier signal and thereby have the means to generate andtransmit the required information signal. Specifically, tags such asthose described in GB 2,112,607 and GB 2,157,132 which require thesustained presence of the excitation field in order to provide aresponse signal, will cease transmitting immediately once the excitationsignal falls below a certain level. Since the interrogator in thesesystems expects to sample data from the tags in bit-contiguous format,sampled with respect to the interrogator's transmit clock referencesignal, data sampled by the interrogator will be corrupted and thepartial message must be discarded. This is a real practical problem withprevious systems where the level of signal detected by a tag is highlydependent upon the relative orientation of tag and excitation antennae,such that as a tag passes an antenna it may at various times during itspassage go through nulls when it will cease transmission. The problem isespecially acute in systems such as that disclosed in GB 2,112,607 wherethe tag transmits its stored data at a very low rate.

In addition, such prior art interdependence between the frequency of theexcitation signal and the frequency of the transmitted informationsignal imposes practical limitations on the frequency range of signalswhich can be used for interrogation and transmission of the information.The rigid mathematical (harmonic) relationship between the excitationand response signals in systems such as that disclosed in GB 2,112,607further complicates the design of the receiver in the interrogator. Itis almost inevitable that harmonic components of the excitation carriersignals will be present and these must be eliminated from the signalreception channel in order to yield an acceptably high signal to noiseratio.

A further disadvantage of such prior art systems is that the transponderis typically tuned to a specific frequency depending on the particularapplication. For example, communications using low frequencyelectromagnetic signals are ideal in situations where the transponder isenveloped in material which causes significant attenuation of radiosignals, such attenuation being much less pronounced at low frequenciesthan high frequencies. However, such transponders which are receptive tolow frequency excitation signals would be inappropriate for applicationsin which a degree of directionality is required. An example of such anapplication would be the use of a transponder which is receptive tomicrowave signals wherein a portable reader and antenna can be aimed ata target object amongst a group of objects each with an attachedtransponder in order to identify the particular object. A transponderwhich is receptive to microwave signals offers the opportunity to designsuch a directional system and identify transponders at a greater rangethan when using low frequency excitation signals.

In certain applications, where objects with attached transponders movearound a large site or from one site to another, it is possible that atcertain times optimum performance would be achieved using a microwaveexcitation signal yet at other times peak performance would be achievedusing a low frequency excitation signal. A practical example of thisscenario is the identification of air cargo containers. At times thecontainers may be resting on a concrete apron at an airport such thatline-of-sight identification is possible using a microwave excitationsignal, while at other times the containers are in the holds of aircraftor maybe in a warehouse or loading/unloading area which may beconstructed partly of metal. Since microwave signals behave in a mannersimilar to light, problems resulting from reflection of the microwavesignals can give rise to uncertainty about the path of the excitationsignal and potentially cause excitation of more than one transponderinadvertently.

A second disadvantage associated with the single excitation frequency ofthe prior art transponders is that, for general applications, governmentregulatory authorities do not typically grant exclusive use ofpredetermined operating frequencies to the user of a transponderidentification system. This means that the users must operate in certainapproved frequency bands which are often shared with, for example, lowpower telemetry systems. As such, the transponder identification systemis subject to interference from other signals which may originate fromintentional radiators also operating in a particular frequency band orfrom high powered equipment generating spurious signals which fallwithin the frequency band used by the transponder. Conversely, atransponder identification system operating in such a band may causeinterference to other systems sharing the same band width. Thus, it maynot be possible to install a transponder identification system at aparticular location because of interference from external signals in thesame frequency band, resulting in severe degradation in performance ofthe identification system, or the risk of causing interference to theother systems.

A further prior art electronic identification system is disclosed in FRA 2 604 808 (Bazin) which teaches the use of a radio electric receiverwhich waits to receive a coded signal which is adequate to start up atiming processor. When the latter is activated, it activates a radioelectric emitter, then activates a memory-reading processor, waits forthe end of the information transfer, stops the radio electric emitterand waits for a fraction of a second before returning to stand-by.However, as with the prior discussed references, there is no teaching inthe Bazin reference of generating an information signal at a frequencywhich is neither derived from nor related to the frequency of theelectro-magnetic excitation field.

It is an object of an aspect of the present invention to provide anelectronic identification system comprising:

a) means for generating an electromagnetic excitation field at a firstvariable frequency,

b) portable transponder means for storing variable identification data,and for transmitting at a second variable frequency an informationsignal containing said data upon entering said field, said transmittingof said signal being independent of said transponder means remaining insaid field, said second variable frequency being neither derived fromnor related to said first variable frequency of the electromagneticexcitation field,

c) means for receiving said information signal and in responsegenerating an output signal representing said variable identificationdata contained in said information signal.

It is also an object of an aspect of the present invention to provide atransponder which can produce a response signal whose frequency can beset according to the conditions encountered at a particular site,thereby maximizing the range of applications and sites to which thesystem may be applied without modification of the transponder'scircuits. This can be seen to have further advantage, where in theabsence of internationally acceptable operating frequencies for thesetransponder systems, it would be possible to have a transponder attachedto an intermodal container, for example, identified at the point ofloading onto a ship in one country, with the transponder set to producea response signal at frequency f₁, say, then on arrival in anothercountry instructed to change the frequency of its response channel sothat it could be identified by receiving signals at frequency f₂, say,where neither frequency f₁, or f₂, were acceptable in both origin anddestination countries.

According to the present invention, an electronic identification systemis provided having a portable transponder for generating andtransmitting an identification or information signal independently ofcontinued presence of the interrogation signal. Thus, the system of thepresent invention may be operated with interrogation and informationsignals from virtually any frequency over the entire electromagneticspectrum (e.g. ranging from DC at the low frequency end of the spectrumto light frequency at the high frequency end). In addition, the systemof the present invention may be utilized in applications where there isfast relative movement between the transponder and the interrogator.

According to a preferred embodiment of the invention, a transponder isprovided which is selectively responsive to various excitationfrequencies, thereby facilitating performance optimization according topredetermined applications and operating specifications.

According to an additional aspect of the invention, digital signalinputs are provided for the transponder, in order to sample externalinputs and report back the results along with the identificationinformation, thereby permitting remote monitoring of the state ofdigital signals representing physical variables such as temperature orpressure.

According to a further aspect of the invention, the information signalmay be encrypted prior to transmission using a pseudo-random varying keygenerated automatically by the tag.

In addition, the transponders of the present invention arereprogrammable during use, by means of modulated radio signals generatedby the interrogator.

Furthermore, the tag according to the present invention formats andtransmits data at standard rates, the precise nature of which is aprogrammable feature of the tag, which means that a reader may in itssimplest form comprise the excitation signal generator (transmitter) andsignal receiver alone without there being the need for a data processingelement (e.g. a microprocessor). Post demodulation, the format of datareceived from a tag is such that it can be presented directly to acomputer via a serial interface, operating in either a synchronous orasynchronous reception mode.

This is a radical departure from previous systems in which the rate andformat of the data transmitted by a tag are such that it must beprocessed (either in format and/or rate) prior to being offered to acomputer via a standard serial interface.

A preferred embodiment of the invention will be described in greaterdetail below with reference to the following drawings, in which:

FIG. 1 is a block diagram of the identification system according to thepresent invention;

FIG. 2 is a block diagram of the transponder or tag in accordance with afirst embodiment;

FIG. 3 is a partial schematic/block diagram of the transponderillustrated in FIG. 2;

FIGS. 4a-4e illustrate the character and message composition of theinformation signal transmitted by a tag, in accordance with the presentinvention;

FIG. 5 is a block diagram of the transponder on tag in accordance withthe preferred embodiment; and

FIG. 6 is a partial schematic block diagram of the transponderillustrated in FIG. 5.

Turning to FIG. 1, a block diagram of the identification system of thepresent invention is shown in its most general aspect. The systemcomprises an interrogator/reader (herein referred to as reader 1) and aplurality of transponders (herein referred to as tags 3) which areattached to objects to be identified (e.g. animals, people, vehicles,etc.). The reader 1 generates an excitation field by means of a radiofrequency (RF) transmitter 5 connected to a transmit antenna 7.Respective ones of the tags 3 are adapted to transmit the response orinformation signals for, among other things, identifying the associatedobject.

The reader 1 receives the information signals by means of a receiveantenna 9 connected to an RF receiver 11. The received informationsignals are decoded and converted into a suitable output signal by meansof microcontroller 13 and serial input/output circuit 15 for display toa human operator, or for use by a computer or other monitoring equipment17.

As will be discussed in greater detail below, each of the tags 3 is inthe form of a miniature radio frequency receiver and transmitter, whichcan store and transmit the coded information signal upon detecting theexcitation signal, for conveying the identity and other characteristicsof the associated object to the reader 1. The circuitry of the tags 3are sealed in respective small, light weight plastic cases. Informationstored in the tags 3 may be altered by means of variable control signalsgenerated by transmitter 5 and antenna 7 under control ofmicrocontroller 13, and may be modified by a user according to specificindividual requirements. Furthermore, the tags 3 may, if required, bewrite-protected so that the data may not be changed.

The microcontroller 13 of reader 1 detects, analyses and formats theinformation signal received from the tags 3, and presents error freeinformation in a variety of formats, as required by each individualapplication.

Serial input/output port 15 allows for direct connection of the reader 1to a computer or other monitoring equipment 17. In addition, reader 1 isprovided with output ports which are electrically and functionallycompatible with industry standard identification and data capturesystems such as magnetic card reader heads and bar code wands, inaddition to the duplex serial communications port 15 for connection to acomputer. Thus, these special, dedicated output ports allow for easyintegration of the reader 1 into existing identification and datacapture systems without modification to the system software.

For the purposes of describing a practical system, the RF transmitter 5shall be a low frequency (LF) transmitter. Antenna 7 is in the form of asimple tuned or untuned wire loop which generates an electromagneticfield for exciting the tags 3. As discussed above, the transmit antenna7 is driven by transmitter 5 and is connected to the reader 1 preferablyby means of a screw terminal connector for convenience and ease ofinstallation. The size of the loop is determined by each application,and may be as small as approximately 50 mm diameter where only shortrange operation is required, or may be wound round a door frame or othersimilar sized aperture in applications requiring the identification ofpersonnel.

Microcontroller 13 is configurable for operation in many user definedmodes, as required for each application. The configuration informationis stored in an electrically erasable, non-volatile memory (E² PROM)under control of microcontroller 13. For example, the reader 1 may beconfigured to transmit information every time it reads data from a tag3, or alternatively to transmit data from a particular tag 3 once onlyfollowing an initial detection.

In operation, the reader 1 essentially emulates a card reader or barcode wand according to well known techniques, and generates an outputsignal in the form of serial logic presented via integrated outputports.

The reader 1 may be powered by a regulated or an unregulated DC or ACsupply, in a well known manner. The power supply 19 regulates the inputpower signal and derives a local +5 V DC signal for internal logiccircuitry (e.g. microcontroller 13) as well as a +12 V DC signal for theanalogue circuits (e.g. transmitter 5, receiver 11).

Turning to FIG. 2, a representative transponder or tag 3 is showncomprising a detector 21 connected to a control circuit 23 which in turnis connected to a transmitter 25. As discussed above, an importantaspect of the present invention is the independence in operation of thedetector 21 and transmitter 25. Specifically, the transponder or tag 3is designed to transmit an information signal via transmitter 25 upondetecting the excitation field by means of detector 21. Yet, inaccordance with the invention, transmission of the information signal isindependent of sustained presence of the transponder or tag 3 within theexcitation field.

Furthermore, the transmitter 25 according to the present inventiongenerates an information signal imposed on a carrier frequency which isneither derived from nor related to the frequency of the excitationfield sensed by detector 21, in contrast with the prior art systemsdescribed in European patent application 0,253,368 and United Kingdompatent applications GB 2,202,981; 2,112,607; and 2,157,132.

In its basic form, the electronic circuitry of the tag 3 is protected bya thin, resilient plastic case, intended to provide protection againstaccidental damage. According to a successful prototype of the invention,the plastic case measures less than 50×40×7 mm and weighs less than 20grams. The case is sealed rendering it impervious to the ingress ofmoisture and dust particles.

A plurality of digital inputs 27 are also provided for monitoring thestate of digital input signals from external sensors such astemperature, pressure, or simple switches, and a plurality of controloutputs 29 are provided (only two such outputs being shown forconvenience) for optional control of external devices connected to thetag 3 (e.g. an LED for confirmation of reprogramming, audible alarmbuzzer, etc.). These outputs may be actuated in a predetermined mannerunder the control of the program stored in the tag's control circuit, oralternatively following receipt of an instruction from the reader 1.

The sampled input information is reported to the reader as part of theinformation signal transmitted via transmitter 25. According to thisfeature, additional applications of the identification system may beprovided without re-engineering the circuits at the heart of theidentification system.

The tag 3 is shown in greater detail with reference to FIG. 3 comprisinga receive antenna 31 forming an inductive loop connected in parallelwith a capacitor 33 for forming a parallel tuned circuit with a resonantfrequency of approximately 135 kilohertz. The receive antenna isconnected to detector 21. Upon entering the excitation field generatedby the reader 1, the detector 21 senses the presence of the field and inresponse generates a carrier detect signal to wake-up logic circuitry 35associated with the control circuit 23. Internal data memory 37 ofcontrol circuit 23 stores user definable information and identificationdata which is encoded into an information signal by means of controllogic 39 and then output to the transmitter 25 as a modulation controlsignal via input/output logic circuitry 41. Operating software for thetag 3 is stored in a program (control) memory 38. The transmitter 25 isenabled by means of a power control signal also received frominput/output logic 41 under control of logic circuitry 39 and inresponse transmits the required information signal by means of atransmit antenna 43, in a well known manner.

According to an important aspect of the present invention, an onboardpower source is provided in the form of a primary cell such as thelithium cell 45 having a nominal terminal voltage of 3 V, and beingconnected to control logic circuitry 39. The provision of an on-boardpower source means that the tag 3 is not required to extract operatingpower from the excitation field, in contrast with the prior art systemdisclosed in United Kingdom patent application GB 2,163,324. The controllogic circuitry 39 monitors the condition of the lithium cell 45 andreports the amount of charge to the reader 1 in the transmittedinformation signal along with the user specified data contained inmemory 37, such that the system monitoring computer 17 is provided withan early warning that the tag 3 is nearing the end of its useful life,and will therefore require replacement.

In operation, the tag 3 remains in a dormant or quiescent state when itis out of range (e.g. more than 3 meters away) from the excitationantenna 7 of reader 1. In the idle state, the tag 3 consumes negligiblepower from the lithium cell 45, and transmitter 25 is down-powered bythe control circuit 23.

The excitation antenna 7 is driven by transmitter 5 for generating alocalized electromagnetic field which is detected by the tag 3 by meansof antenna 31 and detector circuit 21. In response to detection of theelectromagnetic field and generation of the carrier detect signal bydetector circuit 21, wake-up logic circuitry 35 enables the controllogic 39 to retrieve the necessary user specified data in memory 37 andenables input/output logic circuitry 41 to generate the requiredinformation signal for transmission via transmitter 25. As discussedabove, the control circuit 23 also monitors the charge state of lithiumcell 45 by briefly enabling an on-board micro-power voltage comparator(not shown), and responsive to the result of comparison generating astatus bit for inclusion in the transmitted information signal.

Unlike prior art transponder identification systems, the tag 3 of thepresent invention is programmed such that it requires only a very briefexcitation signal for enabling it to transmit the information signal.This aspect, coupled with the use of an efficient UHF transmitter 25eliminates the necessity in prior art systems of requiring the sustainedpresence of the excitation signal for the duration of transmission ofthe information signal. Consequently, it is possible according to thepresent invention to identify objects which are moving at a high speedrelative to antenna 7, without requiring the antenna 7 to be ofunmanageably large dimensions.

According to the preferred embodiment, the tag 3 stores up to 32 bytesof user specified data within memory 37. Of course, the number ofcharacters stored is limited only by storage capacity of memory 37. Thisdata relates to information about the object to which the tag 3 has beenattached. The data is stored in a "free field" format, which means thatthe data may be interpreted in different ways according to eachapplication.

The data stored in memory 37 is reprogrammable by the user, by means ofsending coded, modulated radio signals to the tag 3 from the reader 1via antenna 7. Programming of the tags 3 does not require contact withthe reader 1 to perform this function.

Programming of the tag is initiated by generation of a keyword signal byreader 1, followed by programming instructions, and, if required,additional user specified data. The tag 3 compares the received keywordwith a stored version of the keyword, and in the event the comparisonfails to match the keywords, the tag 3 denies access to memory 37. Inaddition to this first level of programming security, the reader 1 andtag 3 may exchange instructions and responses in a rigidly defined"hand-shake mode" such that if this mode is not followed, the tag 3 willagain refuse access to the memory 37.

It is possible to write protect the tags 3 by incorporating a "read onlyflag" associated with the control logic circuitry 39 for rendering thetag 3 effectively "read only" such that it may not be reprogrammed bythe user. The read only flag is analogous to a read only attributeappended to a file stored on a computer disc according to well knownprior art.

After programming data has been sent to the tag 3 for storage in memory37, the reader 1 may assert the read only flag such that the tag 3 willthereafter not allow the data stored in memory 37 to be altered.However, this protection may be removed by an instruction generated bythe reader 1 for over-writing the write protection.

The tag 3 may also be programmed to generate a unique (characteristic)information signal upon egress from the excitation field so that thereader 1 is notified that the object bearing the tag has exited from thefield of influence of the excitation signal.

In order to accommodate different applications, the identificationsystem of the present invention is designed to form a turnkey system foran end user. For example, in a personnel identification system it is aprerequisite that many tags 3 can be identified at the sametime--simultaneous identification--yet the identification process can berelatively slow (e.g. 100-200 milliseconds). In contrast with thepersonnel identification application, identification of a fast movingobject such as a car will typically not require simultaneousidentification. Thus, each tag 3 can be configured to optimize itsperformance for any applications by simply programming the tag 3 withspecial instructions via the reader 1. Table 1 lists the programmablevariables for the tags 3 which may be modified by the reader i to tailorthe performance of the system to each individual application.

                  TABLE 1                                                         ______________________________________                                        Parameter                   Range                                             ______________________________________                                        Number of data bytes transmitted per message                                                              1 to 32                                           Number of initial message transmissions immediately                                                       1 to 15                                           following detection of the excitation signal                                  Delay between detection of the excitation signal and                                                      1 to 15                                           transmission of initial message packets, measured                             as a multiple of a single message duration                                    Interval between message transmissions subsequent to                                                       1 to 250                                         initial message transmissions, measured as a multiple                         of a single message duration                                                  Number of repeated single message transmissions after                                                      0 to 255                                         transmission of set number of initial messages                                                            or                                                                            unlimited                                         Transmit data rate (bits per second)                                                                      4800 to                                                                       38400                                             Burst mode (send set number of messages following                                                         yes or no                                         detection of the excitation signal, regardless of                             presence or absence of that signal)                                           Encrypt data prior to transmission                                                                        yes or no                                         Write protect user-data     yes or no                                         Transmitted data format     sync                                                                          or async                                          ______________________________________                                    

As can be seen from Table 1, some notable features of the presentinvention include initial pseudo-random delay, variable interval betweensuccessive retransmissions, variable number of retries and dataencryption prior to transmission- The features of initial pseudo-randomtransmission delay, variable interval between successiveretransmissions, and variable number of retries are used to accommodatesimultaneous detection of information signals in applications such aspersonnel identification.

Simultaneous detection of information signals from numerous transpondersis known in the prior art. Some systems, such as those disclosed inUnited Kingdom applications GB 2,202,981 and GB 2,157,132 usesophisticated polling schemes in order to resolve contention betweentransponders competing for a single channel communication link to thereader or interrogator. However, such prior art techniques suffer fromthe disadvantage of requiring two-way (i.e. bidirectional) communicationbetween the transponders and reader in order to achieve this objective,thereby reducing the speed of arbitration and hence degrading therelative speed of movement accommodated between the transponders andreader. However, in practice, only a limited number of transponders(i.e. objects to be identified) can be accommodated within range of theexcitation signal generated by the reader. This physical limitdetermines the maximum number of transponders which can be identifiedsimultaneously. According to the present invention, as discussed abovewith reference to Table 1, each transponder or tag 3 can be programmedto delay its initial transmission by a variable amount, such that thelikelihood of another transponder or tag 3 broadcasting at the same timeis reduced. Hence, the likelihood of the signal from one tag 3 beingreceived uncontested is substantially improved. This initial signaldelay technique of the present invention does not require communicationfrom the reader 1 to the transponder during the identification processand therefore does not significantly extend the length of the timerequired to identify several transponders.

In some applications, where it is known that the tags 3 will remain inthe excitation field for a considerable period of time relative to thelength of time taken to receive an identification message, it may bedesirable to ensure that the tags 3 fall idle once identified and do nottransmit further identification messages, thereby clearing thecommunications channel to the receiver 11 of reader 1 for use by othertags 3 entering the excitation field at a later time. The tag 3 of thepresent invention supports this function by means of programmability tooffer a set number of retransmissions whilst it experiences a continuousexcitation signal from the reader 1, thereby giving a furtherimprovement to the simultaneous identification performance of thesystem. In addition, the value of the retransmission interval stored ina tag can be extended by a pseudo-random amount so that the periodbetween a pair retransmissions (e.g. Ri and Ri+1) is different than theperiod between the retransmissions Ri+1 and Ri+2. This simple techniqueeffectively results in the ability to accommodate an unlimited number oftags 3 in the field of influence of the excitation signal, and each tag3 entering the excitation field will be quickly identified withoutrecourse to a complex two-way communication scheme as discussed in theprior art.

The use of keys for encryption and decryption is also well known in dataencryption communication systems (e.g. military radio systems).According to such systems, the key is changed in a random fashion andapplied to messages prior to transmission and on reception. The value ofthe key at any time is known to both the transmitter and receiver, andthe algorithm under which the key is changed, and the encryption anddecryption algorithms are also known to both the transmitter andreceiver. Thus, once synchronization is achieved between the receiverand transmitter (e.g. the key is set to an initial value), the key canthen be changed frequently and without making the key publicly known.This prior art type of system assumes that there is considerableintelligence in the form of a powerful microprocessor in both thetransmitter and receiver.

Encryption has been applied to transponder identification systems aswell. The broad ranging applications for identification transpondersmeans that a tag might hold information in its memory that is either ofitself sensitive, or the tag may be used for security access controlpurposes. In either case the system would benefit from an added degreeof protection for the information transmitted by a tag, such thatfirstly the actual information contained in data patterns transmittedwas not obvious, and secondly that electronic eavesdropping usingsophisticated equipment to receive, reconstitute and re-broadcast atransmission would not, in the case of a security application, grantaccess to secure areas or information. The schemes devised for thecurrent invention gives both facilities to the system.

One approach is disclosed in United Kingdom application GB 2,202,981, inwhich a key is broadcast to the transponder by the reader, which thetransponder uses to encrypt its stored data prior to transmission. Thisapproach suffers from the principal disadvantage that there is therequirement of two way communications between the reader andtransponder, since the reader must give the key to a transponder inorder that an encrypted message may be formatted and broadcast. This inturn means that the relative speed of movement between transponder andreader is reduced by comparison with the simple excitation/responsesequence of the present invention, as discussed herein above.

In accordance with an additional feature of the present invention, dataencryption is provided without the requirement to pass a key from thereader 1 to the transponder 3. Specifically, according to the invention,the tag 3 generates the encryption key itself, using a definedalgorithm, such that the key varies in a pseudo-random fashion with avery long cycle length. The key is then used to encrypt the data storedin memory 37 prior to transmission. However, in order that the reader ican make use of the information and apply the correct key in order todecipher the received information signal, the key is embedded in thebroadcast information signal from tag 3.

In the reader 1 of the present invention, a specific location isreserved in the memory of microcontroller 13 for storing the lastreceived value of the encryption key. Subsequent transmissions by thesame tag 3 will report a different key value. The reader 1 qualifies areceived message by inspecting the last value of the encryption keyassociated with a particular tag 3, and if the key is different from thenewly received value, then the message is regarded as valid whereas, inthe event the keys are identical, the information signal is rejected(e.g. the bearer of the tag 3 would be denied access in a personnelidentification system). This scheme prevents the recording and replay ofa signal transmitted by a tag in an attempt to breach the security ofthe system, since the second (false) transmission will be treated asinvalid by the reader.

FIG. 4 shows the typical character and message composition forinformation transmitted by tag 3.

Specifically, FIG. 4a shows an asynchronous serial frame, comprising astart bit (S), eight data bits (DO-D7), and a stop bit (P). The startbit is always logic 0 polarity, and the stop bit is always

logic 1. The data bits may be of either state. The

start bit is transmitted first (in time) and the stop bit last. Theduration of each bit is the same. This

is a standard format used in serial data communications betweencomputers and peripherals. The UHF receiver 11 of the reader 1 commencessampling with respect to the leading edge of the start bit, the validityof which is qualified by sampling the state of the start bit at itsnominal centre in order to eliminate false start bits Caused by noise inthe reception channel. Each data bit is sampled at its nominal centre,so the accuracy of each bit period need only be sufficiently good toensure proper sampling over the length of one frame. Hence there is noneed to provide a self-clocking modulation scheme with this type offrame format. By way of example, the practical implementation for thistype of coding is effected in the preferred embodiment of the presentinvention by means of a frequency modulated transmitter 25 (FIG. 3),such that one logic state (logic 1, say) is transmitted at a frequency[f₀ +f_(s) ], where f₀ is the fundamental frequency of the transmitter'sreference oscillator, and the change in frequency caused by taking themodulation control signal from logic 0 to logic 1 is 2 f_(s). In thispractical example, a frequency shift keyed (FSK) modulation scheme isimplemented.

FIG. 4b shows a synchronous serial frame, which comprises just eightdata bits (D0-D7). Unlike the asynchronous frame there is no start orstop bit. Like the asynchronous frame, data bit D0 is transmitted first(in time) and D7 is transmitted last. The synchronous frame format isalso widely used in computer communications. Unlike the asynchronousframe format, synchronous transmissions require either that thefrequency of the transmit reference clock and receiver sampling clockare very closely matched, or (and more commonly) that a self-clockingscheme is employed. Using the above-mentioned FSK modulation scheme, itwould be preferred that each data bit was represented by a portion ofthe upper transmission frequency [f₀ +f_(s) ] and a portion of the lowertransmission frequency [f₀ -f_(s) ]. An example of a practical schemewould be to use a so-called Manchester encoded scheme, where each bit ofa character was transmitted such that the first half of a bit wastransmitted at [f₀ +f_(s) ] and the second half of the bit wastransmitted at [f₀ -f_(s) ] if the state of the data bit was logic 1, or[f₀ -f_(s) ] followed by [f₀ +f_(s) ] if the state of the data bit waslogic 0. Hence the receiver can extract a clocking (sampling) signal ona bit-by-bit basis, thereby maintaining sampling timing accuracy of themessage length.

As described herein above, one programmable feature of the presentinvention permits the tag 3 to transmit its stored data in either ofthese formats, character by character, with the speed of transmission(the bit period) also being programmable to match standard datacommunications speeds.

FIG. 4c shows a string of "n" characters, each composed in anasynchronous serial frame. There may be any number of characters betweencharacter "a" and end character "n". It should be noted that there isnot of necessity contiguity between successive characters, and theinterval between character frames may vary. The interval betweencharacter "b" and character "n" is filled with other characters, withintervals between those characters which may be variable.

FIG. 4d shows a string of "n" synchronous character frames, composed toform a synchronous character string. It should be noted that there is nointerval between successive characters in this mode, and that theinterval between character "c" and character "n" in FIG. 4d is aninteger multiple of the time taken to transmit one character and will inpractice be filled with contiguous character frames.

Referring now to FIG. 4e, a message composition is shown which isindependent of the frame format and method of modulation. Each character"a" to "n" is either an asynchronous character frame or a synchronouscharacter frame, and each block in FIG. 4e which represents a singlecharacter frame is deemed to include any framing (start and stop) bitspresent (in the case of an asynchronous frame format).

It is a programmable feature of the present invention that the length ofa message comprising "n" character frames is variable; that is "n" is aprogrammable variable. As discussed herein above, the information storedin the tag's data memory 37 is held in a "free-field" format, whichmeans that the contents of the memory is variable and open tointerpretation which will depend upon the application of the tag. Thatis to say the user of the system can store in the tag's memory 37 anyinformation, held in 8 bit frames, so that it is quite practical tostore in locations for characters "a" and "b" two synchronizationcharacters which are required to allow a receiver operating in asynchronous reception mode to discern the start of a message from a tag,for example. Similarly error checking information would typically betransmitted in character positions "n-2", "n-1", with a closing flag inposition "n" in accordance with a standard synchronous data linkprotocol.

Furthermore, as discussed herein above, it is an additional feature ofthe present invention that tag 3 may be configured to apply anencryption algorithm to the data stored in its memory 37 prior totransmission, and that in the scheme devised the encryption key isincluded in the message transmitted by the tag. The encryption key isembedded in a transmitted message in a location known to the reader 1(which may be variable), so that the reader may decipher a receivedmessage which has been encrypted.

Additional structural and operational details of the tags 3 are providedbelow with reference again to FIG. 3 of the drawings, comprising acurrent best mode of the invention. Of course, additional modes andembodiments are possible within the scope of the present invention.

Field Detector 21

Low frequency signal receiver 21 comprises a ferrite cored inductor 31,which is brought to resonance at the frequency of the excitation signal,by parallel tuning capacitor 33. The output signal from this tunedcircuit is applied to the base of a transistor 34. Sufficient biascurrent flows into the base of transistor 34 to cause it to conduct whenthe voltage developed across the tuned circuit exceeds approximately 600mV. The collector of transistor 34 is connected to an amplitudedemodulator comprising resistor R1 and integrator capacitor C1, so thata signal detected by antenna 31 will cause a signal to be passed to thecontrol circuit 23 indicating the presence of an excitation signal.

Control Circuit 23

The control circuit 23 in this practical example is an 8-bit CMOSmicrocontroller and comprises mask-programmable ROM (program memory 38),RAM (data memory 37) a timer and data processing circuitry (controllogic 39), input/output ports (input/output logic 41), and operatingmode control logic (wake-up logic 35). It is continuously powered fromthe lithium cell 45 which has a nominal terminal voltage of 3 V (for alithium manganese-dioxide cell). Control circuit 23 derives its internaltiming from an on board oscillator, which has a ceramic resonator X1 asits frequency determining element.

Whilst the tag 3 is remote from the influence Of the electromagneticfield generated by the transmit antenna 7 of the reader, the tag is in aquiescent stage, in which it consumes minimum current from its lithiumcell 45. The control circuit is in a "sleep" stage, with its oscillatorand the UHF transmitter 25 disabled. In this "sleep" mode, the controlcircuit 23 is responsive to a logic low signal from the field detectorcircuit 21. When the voltage on the collector of 34 goes low, thecontrol circuit is brought out of the sleep mode and its oscillator isrestarted. During the short period whilst the oscillator is stabilizing,the control circuit is prevented from executing instructions by internallogic. The control circuit 23 will power-up and enable the UHFtransmitter 25 periodically, and produce response signals by applyingmodulation control signals via an output pin. The composition of thedata and the rate at which it is presented via the modulation controlline is determined by the information stored in the data memory 37 ofthe control circuit.

When the tag 3 no longer experiences an excitation signal detectable bythe field detector 21, the voltage on the collector of 34 rises, and thecontrol circuit may stop transmission of response signals, then placeitself in a sleep mode ready for detection of the next excitationsignal. It may be that the control circuit 23 has been programmed byinstructions from reader 1 to ignore the state of the signal from thefield detector circuit 21 until it has completed transmission of anidentification message or messages, in which case there will be a delaybetween the collector of 34 going high and the cessation oftransmissions by the tag 3.

During the period immediately following cessation of transmissions bythe tag 3 and after removal of the excitation signal, the controlcircuit 23 will monitor the output from the field detector circuit 21 ormodulations which might have originated from reader 1, since this is themethod of communicating instructions and data to the tag 3. If correctmodulations are detected, the control circuit 23 enters a receptive modein which it will monitor the carrier detect line for further modulatedsignals. Properly coded signals will cause the control circuit 23 toaccept data for storage in its data memory 37 for subsequenttransmissions.

UHF Transmitter 25

The UHF transmitter 25 is essentially an oscillator with Q1 as itsactive element, which is designed to radiate a very low level of signalthrough transmit antenna 43, which may for convenience be a profiledtrack on the printed circuit board (PCB) on which the other componentsof the tag are mounted. Modulation may be applied to the oscillatorwhich causes the frequency of oscillation to vary slightly, therebyallowing transmission using a frequency shift keyed (FSK) system.

The circuit 25 comprises high frequency transistor Q1, which has aninductive collector load in the form of a PCB track, with feedbackcapacitor C3 between collector and emitter to form a simple Colpittsoscillator. The ferrite bead FB1 is included since it presents animpedance which rises very sharply above the fundamental frequency ofthe oscillator, thereby significantly reducing the harmonic distortionin the output signal. The fundamental frequency of this oscillator isdetermined by a surface acoustic wave (SAW) resonator Fl.

The operation of the transmitter 25 is controlled by the control circuit23, the oscillator is enabled by applying bias directly from an outputpin via resistor R2 to the base of Q1. When the control (bias) voltageis high, the oscillator is enabled. The base voltage is set by doublediode D1 and is thus independent of the supply voltage. When the biasvoltage is low, the oscillator is disabled, and the circuit consumesnegligible current. The output from the oscillator is peaked by variablecapacitor C5, which predominantly affects amplitude and to a much lesserextent frequency.

Modulation is applied to the transmitter 25 by applying a controlvoltage through resistor R3 to diode D2, which, whilst not being a truevariable capacitance diode, exhibits a change in reverse capacitancewith applied voltage sufficient to alter the impedance of the collectorload circuit and pull the frequency of the oscillator. This variableload capacitance is coupled to the collector circuit by capacitor C2. Afrequency modulation scheme is thereby operated by the control circuit.Naturally, a 100% amplitude modulated signal can be generated at one oftwo frequencies simply by gating the base bias on and off at therequired rate.

Turning to FIGS. 5 and 6, a transponder according to the preferredembodiment is shown comprising signal receivers 51 and 52, a demodulator53, a control circuit 54, frequency synthesizer 55, transmit antenna 56and a cell 57.

Detectors 51 and 52 are shown by way of example as being low frequencyand microwave detector circuits, respectively. However, other frequencyspecific detectors may be utilized.

Microwave detector 51 comprises a half-wave dipole antenna in the formof strips on a printed circuit board, radio frequency choke L3 andSchottky detector diode D1. The energy detected is stored as a charge oncapacitor C4, and resistor R2 provides a DC path to the reference levelV_(ss) for diode D1. The voltage developed across capacitor C4 causesbias current to flow into the base of transistor Q2.

Low frequency signal receiver 52 comprises a ferrite cored inductor L2,which is brought to resonance at the frequency of the excitation signalby parallel tuning capacitor C2. The output signal from this tunedcircuit is applied to the base of transistor Q1. sufficient bias currentflows into the base of transistor Q1 to cause the transistor to conductwhen the voltage developed across the tuned circuit exceedsapproximately 600 mV. The collector of transistor Q1 is connected to thecollector of transistor Q2 and to the demodulator 53, so that either asignal detected by a low frequency signal detector 51 or by microwavesignal detector 52 will cause a signal to be passed to the controlcircuit 54 indicating the presence of excitation signal.

In common with the transponder discussed above with reference to FIGS. 2and 3, various programmable features are provided and the operation ofthe transponder may be modified by modulated signal programming.

In the preferred embodiment illustrated, amplitude demodulator 53comprises a resistor R1 and a capacitor C1. Either transistor Q1 ortransistor Q2 is capable of clamping the voltage on the integratorcapacitor C1 in order to indicate the presence of an excitation signalto the control circuit 54.

Circuit 54 controls operation of the transponder. In the absence of asignal from the demodulator 53, the control circuit 54 is in a quiescentstate. The control circuit down powers the frequency synthesizer 55, andplaces itself in a mode where it consumes least current. The controlcircuit 54 exits the quiescent state in response to detection of anexcitation signal. According to the preferred embodiment, controlcircuit 54 is manufactured as a custom-designed integrated circuitfabricated in CMOS technology. Alternatively, the control circuit 54 canbe in the form of a mask programmed 4 or 8 bit CMOS micro controller,such as the 84C00 family of micro controllers manufactured by PhilipsComponents, such a device containing the functional blocks shown in FIG.5 necessary to control the operation of the transponder, including adata memory where identification data may be stored.

The control circuit 54 manages the operation of the frequencysynthesizer 55, via the clock and data lines shown, by passing data tobe transmitted via the modulation control line, and is able to place thesynthesizer in a powered-down state by actuation of a power controlsignal.

Frequency synthesizer 55 comprises principally a crystal controlledreference oscillator and a digital phase locked loop which acts toproduce frequency multiplication, and includes a voltage controlledoscillator. A detailed description of the elements of the frequencysynthesizer is not provided herein, as the design and principles ofoperation of such devices are well known in the industry. For example, awell known frequency synthesizer is the UMA1014 manufactured by PhilipsComponents.

As discussed above, the control circuit 54 provides instructions to thefrequency synthesizer 55 via the clock and data control lines, includingthe setting of the transmit frequency. Modulation is applied to thefrequency synthesizer 55 from control circuit 54 via the modulationcontrol signal line. This modulation may affect the amplitude, frequencyor phase of the transmitted signal, which is propagated by transmitantenna 56 for reception by a receiver which is tuned to thetransmission frequency of the transponder.

The manner in which the transponder is programmed, for example to directthe control circuit 54 to pass an instruction to the frequencysynthesizer 55 to select a particular response frequency, is discussedabove in greater detail.

The transponder circuit is powered by a small primary cell 57, which mayuse lithium-manganese dioxide or lithium-thionyl chloride chemistries,for example.

In summary, according to the present invention, an identification systemis provided in which sustained presence of the tag or transponder is notrequired for transmission of an identification information signal.Independence between the receiver and transmitter of the tag permitstransmissions of data at standard rates and with standard formats, andpermits the concepts described to be applied to any system independentof the frequencies of operation. Additional powerful features such asdata encryption and the programmable characteristics of the tagsdescribed are features in isolation and not reliant upon the separationbetween receiver and transmitter.

Other embodiments and variations of the present invention are possiblewithin the sphere and scope of the claims appended hereto.

For example, the number of signal receivers

detectors in the preferred embodiment of FIGS. 5 and 6 need not belimited to only two as shown. Theoretically, an unlimited number ofexcitation signal receivers can be employed allowing the transponder tobe excited by a wide range of excitation frequencies. An alternativeimplementation could use a signal receiver which scans across a range offrequencies.

We claim:
 1. An electronic identification system comprising:a) means forgenerating an electromagnetic excitation field at a first frequencysettable to a plurality of different frequencies; b) portabletransponder means including means for storing variable identificationdata, and means for detecting ingress of said portable transponder meansinto said electromagnetic excitation field and in response transmittingat a second frequency settable to a plurality of different frequenciesan informational signal containing said data, said transmitting of saidsignal being independent of said transponder means remaining in saidfield, said second frequency being neither derived from nor related tosaid first frequency of the electromagnetic excitation field, and saidfirst and second frequencies remaining constant during transmission ofsaid information signal; and c) means for receiving said informationalsignal and in response generating an output signal representing saidvariable identification data contained in said informational signal. 2.The system of claim 1 wherein said transponder means is adapted forattachment to an object to be identified, and further comprises:a) areceiver tuned to said first frequency for detecting said field, b) amemory for storing said variable identification data, c) logic means forretrieving said data from said memory responsive to said receiverdetecting said field and in response generating said information signal,d) a transmitter for receiving said information signal from said logicmeans and in response transmitting said information signal at saidsecond frequency.
 3. The system of claim 2 wherein said transpondermeans further comprises a plurality of external input ports forreceiving external input data, and means for sampling said externalinput ports and in response encoding and transmitting said input data aspart of said information signal.
 4. The system of claim 1 furthercomprising circuitry within said transponder means for encrypting saidinformation signal prior to transmission thereof, and circuitry withinsaid means for receiving for decrypting said information signal prior togenerating said output signal.
 5. The system of claim 1 wherein saidtransponder means is adapted to transmit a further information signalresponsive to said transponder means egressing from said field.
 6. Thesystem of claim 1 wherein said transponder means is programmable fortransmitting said information signal in accordance with standard dataformat and data rates.
 7. The system of claim 1 wherein said transpondermeans is adapted to delay transmission of said information signal by apseudo-randomly varying amount upon entering said field.
 8. The systemof claim 1 wherein said means for generating said electromagneticexcitation field comprises a low frequency (LF) radio transmitterconnected to a loop antenna.
 9. The system of claim 1 wherein said meansfor receiving comprises an ultra-high frequency (UHF) receiver connectedto a microcontroller.
 10. The system of claim 2 wherein said receivercomprises a low-frequency (LF) tuned antenna connected to a passivefield detector circuit.
 11. The system of claim 2 wherein saidtransponder means further comprises a battery power source and means formonitoring charge in said battery power source and incorporating datarepresenting said charge in said information signal.
 12. The system ofclaim 1 wherein said means for generating is adapted to modulate saidexcitation field in accordance with a variable programming signal forprogramming said memory responsive to said transponder means enteringsaid field.
 13. The system of claim 4 wherein said circuitry within saidtransponder means further comprises means for generating a pseudo-randomvarying key associated with each transmission of said informationsignal, said key being generated in accordance with a variablealgorithm, means for encrypting said information signal using said key,and means for embedding said key in encrypted information signal. 14.The system of claim 13 wherein said circuitry within said means forreceiving further comprises means for locating said key in saidencrypted information signal and in response decrypting said informationsignal using said key.
 15. The system of claim 14 wherein said circuitrywithin said means for receiving further comprises means for storingsuccessive values of said key and comparing each said key located insaid information signal with the prior stored key, and in the event thecompared keys are identical the decrypted information signal is rejectedwhereas in the event the compared keys are different the decryptedinformation signal is accepted.
 16. The system of claim 1 wherein saidtransponder means is adapted for attachment to an object to beidentified, and further comprises:(a) a receiver for scanning a range offrequencies in order to detect said electromagnetic excitation field atsaid first frequency, (b) a memory for storing said variableidentification data, (c) logic means for retrieving said data from saidmemory responsive to said receiver detecting said field and in responsegenerating said information signal, and (d) a transmitter for receivingsaid information signal from said logic means and in responsetransmitting said information signal at said second frequency.
 17. Asystem of claim 1 wherein said transponder means is adapted forattachment to an object to be identified and further comprises:(a) aplurality of receivers tuned to respective frequencies for detectingsaid field at a predetermined one of said respective frequencies, (b) amemory for storing said variable identification data, (c) circuit meansfor retrieving said data from said memory responsive to detection ofsaid field at said predetermined one of said frequencies and in responsegenerating said information signal, and (d) a transmitter for receivingsaid information signal from said circuit means and in responsetransmitting said information signal at said second frequency.
 18. Thesystem of claim 17 wherein said plurality of receivers comprises amicrowave detector and a low frequency signal detector, respectively.19. The system of claim 18 further comprising a demodulator fordemodulating signals received by said microwave detector and lowfrequency signal detector, respectively.
 20. The system of claim 19further comprising a frequency synthesizer connected to said circuitmeans for receiving control signals from said circuit means and inresponse generating and transmitting said information signal at saidsecond predetermined frequency as specified by said control signals. 21.The system of claim 1 wherein said first frequency is selectable foroptimum performance dictated by application, and wherein saidtransponder means is adapted for attachment to an object to beidentified, and further comprises:(a) an adaptive receiver for detectingsaid first frequency, b) a memory for storing said variableidentification data, c) logic means for retrieving said data from saidmemory responsive to said receiver detecting said first frequency and inresponse generating said information signal, d) a transmitter forreceiving said information signal from said logic means and in responsetransmitting said information signal at said second frequency.